Material treatment by cryogenic cooling

ABSTRACT

A method of treating materials, particularly metals such as steel, cast iron, and the like, by cryogenic cooling for altering the microstructure of the materials for improved resistance to wear, to corrosion, and the like, including the steps of reducing the material to a predetermined low temperature at a preselected uniform rate below a rate which will cause thermal fracturing within the grain boundaries, holding the materials at such low temperature for a substantial period of time depending upon the material characteristics and other features, and thereafter permitting the temperature of the material to return to normal. The steps are carried out by supporting the material above a body of cryogenic fluid and incrementally bringing the material and the fluid together by either lowering the material into the fluid or by raising the body of fluid to envelop the material for the stepwise temperature reduction of the material, emersing the material in the fluid to produce the desired minimum low temperature, holding the material in the cryogenic fluid for the predetermined substantial length of time at which the temperature of the material is to be maintained at such low temperature, and lifting the material from the fluid or permitting the fluid to boil off and thereafter allowing the material to return to room temperature.

United States Patent n91 Lance et al.

[ June 24, 1975 [75] Inventors: James W. Lance; Henry M. Jones,

both of Stonewall, La.

[73] Assignee: The Mangrove Enterprise,

Incorporated, Wilmington, Del.

[22] Filed: Sept. 9, 1971 [2l] Appl. No.: 179,218

[52] US. Cl. 148/125; 62/64; 62/65 [51] Int. Cl. C22c 41/04 [58] Field of Search 62/62, 63, 65, 64; 148/125; 264/348 [56] References Cited UNITED STATES PATENTS 2,l97,365 4/l940 Kjerrman l48/l25 2,949,392 8/1960 Willey l48/l25 OTHER PUBLlCATlONS Boxboard Containers, Cryogenics Extends Knife Life," July 1970.

Controlled Dry Cryogenic Process, Materials Improvement, lnc., Hazel Park, Michigan, Pub. i968.

Iron Age, Aug. 26, l97l, Vol. 208, pgs. 55 & 56. Iron Age, Feb. ll, l97l, pg. 58.

Primary Examiner-C. Lovell Attorney, Agent, or Firml-l. Mathews Garland 5 7 ABSTRACT A method of treating materials, particularly metals such as steel, cast iron, and the like, by cryogenic cooling for altering the microstructure of the materials for improved resistance to wear, to corrosion, and the like, including the steps of reducing the material to a predetermined low temperature at a preselected uniform rate below a rate which will cause thermal fracturing within the grain boundaries, holding the materials at such low temperature for a substantial period of time depending upon the material characteristics and other features, and thereafter permitting the temperature of the material to return to normal. The steps are carried out by supporting the material above a body of cryogenic fluid and incrementally bringing the material and the fluid together by either lowering the material into the fluid or by raising the body of fluid to envelop the material for the stepwise temperature reduction of the material, emersing the material in the fluid to produce the desired minimum low temperature, holding the material in the cryogenic fluid for the predetermined substantial length of time at which the temperature of the material is to be maintained at such low temperature, and lifting the material from the fluid or permitting the fluid to boil off and thereafter allowing the material to return to room temperature.

8 Claims, No Drawings MATERIAL TREATMENT BY CRYOGENIC COOLING This invention relates to methods of material treatment and more specifically relates to extremely low temperature treatment of materials for improving the miscrostructurc of the materials.

Material treatment, particularly of metals, has been approached in a number of ways including the use of processes involving both heating and cooling the materials for effecting various changes in the basic structure of the materials. With certain steels and irons, some of these methods have included heating to very high temperatures followed by cooling to very low temperatures for transformation of the austenite present to martensite. in some other methods similar steps have been used for stress relief. In some instances thermal fracturing within or along the grain boundaries has resulted from excessively rapid cooling rates which produce uneven temperature reductions within the material causing structural failure or substantially increasing the tendency of the material to internal structural failure. Such damaging of the material is the result of uneven temperature rate changes which produce material size changes exceeding the yield characteristics of the material. Additionally, some of the prior methods have produced random distribution of retained austenite formations within the material providing a nonhomogeneous structure which is not uniform in such characteristics as resistance to wear. Such nonhomogeneity may result in failures within the material microstructure along the resulting paths of least resistance. Also, some of the methods of material treatment which have been practiced in the past affect only the surface portions of the material being treated. A major portion of the prior art material treatment processes involve both steps of heating the material and steps of cooling it, the material being first heated and then subsequently quenched or cooled. One known method of material treatment involves substantial cooling followed then by a heating step. The conversion of austenite to martensite has been recognized and is discussed in several of the prior art patents which indicate the transformation may be obtained by severe cooling. The prior art has, however, not practiced or recognized the substantial benefits of incremental cooling of the material and the substantial holding or soaking periods at the minimum low temperature employed by the present invention.

Prior art particles for surface hardening have involved cooling to sub-zero temperatures and subsequent reheating to elevated temperatures in the 300500F. range. Such a method draws the carbon particles to the surface of outer skin of the material being treated thereby providing an increase in the surface hardness of the material, but the process, unlike the method of the present invention, simply produces a surface hardness rather than a uniform refined microstructure throughout the material. Thus, it has not been found that the prior methods of material treatment by cooling produce the internal structural changes inherent in Applicants process.

It is principal object of the invention to provide a new and improved method of material treatment for altering the metallurgical characteristics of the material. It is another object of the invention to provide a new and improved method of material treatment which involves cooling the material. It is a still further object of the invention to provide a method of material treatment which does not necessitate the use of a heating step or steps. It is a further object of the invention to provide a method of treating materials for improving the structural characteristics of the materials which utilizes cryogenic fluids for lowering the material temperatures to extremely low levels. It is another object of the invention to provide a material treatment method which effects a conversion of iron base alloys of the austenitic type to martensitic type structures. It is a further object of the invention to provide a process for treating metal alloys wherein the alloying elements of the material are better dispersed throughout the material. It is another object of the invention to provide a material treatment process which minimizes or eliminates thermal shock problems normally encountered in cooling methods. It is a further object of the invention to provide a material treating process which, when used with carbon steels, does not result in migration of the carbon particles to the surface of the member being treated. it is a further object of the invention to provide a material treating process which, when applied to steel alloys, effects the break-up of the carbides and the redistribution of alloying materials resulting in a more homogeneous microstructure in the material. It is a further object of the invention to provide a material treating process which improves the resistance of the material under impact, against wear, abrasion, and erosion producing factors.

In accordance with the invention, there is provided a method of treatment of materials, particularly irons and steels and the like, by incrementally cooling the materials from normal room temperature to temperatures which may be on the order of about 320F., holding the material temperature at such minimum low value for a substantial period of time such as for about 18 to 30 hours or longer, and thereafter raising the tem perature of the material back to normal room temperatures. The process is preferably carried out by use of cryogenic fluids as the cooling medium.

Specific steps included in the method of the invention, together with its objects and advantages, will be more readily understood and appreciated from the following detailed description of preferred embodiments and specific examples of the method of the invention.

The practice of the specific steps involved in carrying out the various forms of the invention may be performed in a wide variety of apparatus ranging from very crude handling facilities to sophisticated, automated equipment. In early testing stages of development of the invention, a cryogenic fluid, such as liquid nitrogen, was held in an insulated container while the material to be treated was supported with suitable simple tongs above the body of liquid nitrogen and progressively lowered toward and subsequently into the nitgrogen for incrementally lowering the temperature of the mate rial, and thereafter soaking the material in the liquid nitrogen. As the material approaches the surface of the body of nitrogen, its temperature is progressively lowered depending upon the rate at which the material is moved toward the liquid body of the nitgogen. The temperature of the material while being treated during the process is directly related to the distance of the material above the surface of the body of the cryogenic fluid. Any suitable means for measuring such temperature above the treating fluid may be employed. [f desired, quite obviously, much more sophisticated equi ment may be used including known devices for automatically monitoring the temperatures above the cryognic fluid surface and progressively bringing the material being treated and the fluid together at a rate calculated to effect the desired incremental lowering of the temperature of the material. An insulated tank for containing the cryogenic fluid may have remotely controlled supporting tongs observable through glass panels in the tank walls such as currently used in nuclear energy processes which permit the manipulation of desired materials and apparatus from a remote observation point.

Basically, the method of the invention involves the cooling of the material to be treated at a rate which avoids thermal shock sufficient to damage the material, holding the material at the desired low temperature for a very substantial period such as on the order to 18 to 30 hours or longer, and thereafter increasing the tem perature of the material at a predetermined rate back to normal room temperature.

The cooling portion of the method is preferably carried out incrementally or stepwise by reducing the temperature at various specified rates while, additionally, in the case at least of certain materials, stopping for a period of time at certain temperature levels during the temperature reduction. The very careful, deliberate temperature reduction is to achieve temperature stabilization at selected levels and at a uniform rate to equalize the temperature throughout the material before subsequently reducing the temperature to a still lower lever. The rate of temperature reduction is essentially an empirically determined step selected to lower the temperature without producing evidence of thermal shock. Once the rate at which the temperature can be lowered without producing thermal shock is determined, it has not been found that further reducing such rate, or, stated otherwise, lowering the temperature more slowly, and holding at the selected points for longer periods of time particularly improve the process. For at least certain materials, there appear to be two thermal shock thresholds, the first being at approximately lF., while the second is at approximately 250F. These thermal shock thresholds are found to be critical points at which thermal shock of the material occurs during constant temperature reduction if stabilization periods are not provided. As the temperature of the material is reduced, the molecular motion of the particles slows down. The objective of the progressive temperature reduction is to achieve somewhat uniform rates of change of the microstructure by obtaining a substantially uniform rate of reduction of movement of the molecular particles. For example, if the molecular motion on the surface of a particular mass exceeds the rate of change of molecular motion in the center of the mass, factors necessary for thermal shock producing intergranualr separations in the material intergranular be developed.

Among the basic objectives in the treating of certain metals by cooling is the movement or relocation of the carbides and, additionally, the dispersion of the alloying media of the metal. In contrast with the desired slow cooling of the method of the invention, rapid cooling is believed to reduce the molecular action in the outer skin of the mass much more rapidly than the molecular action within the interior of the mass with the difference resulting in either fractures actually developing or, perhaps, a weakening which prematurely pro duces fractures throughout the mass.

The various element carbides are redistributed among other carbide matrices of certain metals by the method of the invention and more finely dispersed while simultaneously a redispersion of the alloying elements of the metals produces a refined microstructure with a more homogeneous mixture of the alloying media. This is considered to be an extension or a continuation of the transformation of austenite to martensite and, thus, the method of the invention is not considered to comprise simply the previously known low temperature austenite to martensite conversion. While such a conversion is inherent to the process of the invention, the prime and basic objective of the process is the redispersion of the alloying materials, and the breaking of the carbides which enhances such redispersion. The short holding periods at the minimum low temperatures, as known in the prior art, do not achieve such redispersion. The term alloying materials" includes tungsten, chromium, molybdenum, nickle, manganese, silicon, sulphur, vanadium, cobalt, and others. With respect to the carbides, some are literally moved around and redistributed while others are broken and the resulting finer carbide particles are dispersed to the material grain boundaries and into other carbide matrices.

One example of the application of the method of the invention is the treatment of an AlSI M2 steel having the following chemistry: carbon0.82 percent; manganese-23.0 percent; silicon-0.25 percent; chromium4.25 percent; molybdenum-50 percent; tungsten-6.25 percent; vanadium-L percent; and the remainder-iron. The mass is supported above a body of liquid nitrogen and lowered toward the nitrogen at a rate calculated to reduce a mass of up to 5 pounds of the steel a first temperature rate of lOF. per minute until the temperature is lowered to lOOF. The rate of temperature reduction is then changed to 6F. per minute until the temperature of the mass reaches 320F. for a period of about 24 hours. The liquid nitrogen is then permitted to boil off while the mass is retained in the cryogenic fluid tank until the mass has reached room temperature. The rate of temperature reduction to the 320F. level is determined on a basis of maintaining at all times the temperature reduction rate at a level which is below or does not exceed the temperature change at which thermal fracture of the mass occurs. It has been found that for masses ranging from about 5 pounds to about 20 pounds it is necessary to adjust the time factor for the cool-down period by an amount of about 30 percent so that such mass is cooled down over a time period of about 30 percent longer than the cool-down time period for the mass weighing up to 5 pounds. Similarly, masses of from about 20 to about lOO pounds have proven to require about 25 percent more cool-down time than the up to 5 pound mass, while masses in excess of pounds require about 40 percent more cool-down time.

An AlSl M3 steel mass of up to 5 pounds is cooled at a rate of [0F. per minute to l00F., at 5F. per minute from l00F. to 250F., and thereafter at 10F. per minute to the soak temperature of 320F. The mass is soaked for 24 hours at -320F. and then permitted to warm to room temperature in the treatment tank after the nitrogen has boiled off.

An AlSl M4 steel mass of up to 5 pounds is treated in accordance with the invention by reducing its temperature at a rate of 10F. per minute to lO0F., at 6F. per minute to 250F., and then at 4F. per minute to 320F. The mass is then soaked for a period of 24 hours at 3 20F. and permitted to return to room temperature in the tank after the nitrogen has boiled off.

An AISI S1 steel mass of up to 5 pounds is first cooled at a rate of 15F. per minute to lF., and then at a rate of 10F. per minute to 320F. The mass is kept at 320F. for a period of 24 hours and then allowed to return to room temperature within the tank after boil off of the nitrogen.

The same size range of masses of A181 S and AlSl S7 steels are cooled at a rate of l5F. per minute to l50F., and then at a rate of 10F. per minute to 320F. The mass is then soaked for hours and returned to room temperature by the same procedure as the above steels.

AlSl types 302, 304 and 316 stainless steels of no more than 5 pounds mass are reduced in temperature at a rate of 10F. per minute to 320F. at which tem perature they are soaked for a period of 24 hours following which the masses are removed from the liquid nitrogen and permitted to warm up in still air. A 347 stainless steel mass of the same weight is reduced in temperature at a rate of 6F. per minute to -320F. and thereafter soaked at such temperature for a period of 24 hours, following which it is permitted to return to room temperature in still air. Similarly, a mass of no more than 5 pounds of a 400 series stainless steel is reduced in temperature at a rate of 5F. per minute to 320F. at which it is then held for a period of 24 hours. The mass is then permitted to return to room temperature after being removed from the liquid nitrogen and supported in still air.

The various types of Monel and lnconel in masses not exceeding 5 pounds are cooled at a rate of 5F. per minute to 320F. at which temperature they are maintained for a period ranging from about 18 to about 24 hours depending upon the size of the particular mass being treated. The material is then warmed up by permitting the nitrogen to boil offin the tank retaining the mass in the tank until it has warmed to a temperature of -100F. and then removing the mass from the tank to still air to permit it to warm to room temperature.

All types of cast iron masses of no more than 5 pounds are cooled at a rate of 5F. per minute to the 320F. level at which the material is held for a period of from 24 to 30 hours depending upon the size of the mass. The mass is then permitted to return to room temperature in the fluid tank after the liquid nitrogen has been allowed to boil off.

Certain differentially hardened members such as roller, ball, and tapered roller bearings are cooled at a rate of 10F. per minute to 100F. at which they are held at constant temperature for 30 minutes. The members are then cooled at a rate of 5F. per minute to 250F. at which they are held for 1 hour and thereafter reduced at a rate of 5F. per minute to 320F. The members are then held at 320F. for a period ranging from 20 to 24 hours following which they are allowed to return to room temperature within the fluid tank after boil-off of the liquid nitrogen.

In each of the specific examples of the above methods. the larger masses are cooled at the increased time rates of 30 percent for masses of about 5 to 20 pounds, 35 percent for masses of about 20 to 100 pounds, and 40 percent for masses in excess of about 100 pounds.

An analysis of the treating of the substantial number of metals discussed by specific example reveals that the incremental rate of cooling or temperature reduction varies from a minimum of about 4F. per minute to a maximum of about 15F. per minute for the masses which do not exceed about five pounds. Adjusting these temperature change ranges for the various mass ranges, such as increasing the time of cool-down by 30 percent for the 5-20 pound mass range, 35 percent for the 20-100 pound mass range, and 40 percent for the over pound mass range size, gives a resulting range of temperature to time increments from the range of a minimum of about 2.85-4F. per minute to a maximum of about l0.7l5F. per minute. Thus, with the materials tested. the slowest cool-down time would be accomplished at a rate of about 2.85F. per minute while the maximum cool-down rate would be at about 15F. per minute. In the case of the stainless steels and Monel and lnconel, the mass was cooled down at a consistant regular rate from room temperature to the 320F. level. It will also be noted that, where it was found necessary to change the rate of cool-down at the two thermal thresholds, the rate was less or lower between l00F. and 320F. than it had been from room temperature to 100F. It will be seen that with two of the AISI grades of steel, the cool-down procedure is effected in three stages, the first to the l0OF. threshold, the second to the 250F. threshold, and the third to the minimum low temperature of 320F. With the other three AlSl grades of steel tested, the cooldown procedure was accomplished in two stages, with the rate of change with two of the samples being made at the 100F. threshold and with the other sample the rate of change being made at the l50F. threshold.

ln photomicrographs of selected samples of metals treated in accordance with the procedure outlined above, it has been found that some of the carbides are moved and redistributed within the structure of the metal while others are broken into smaller particles which are then distributed through the material. The change which is found to be brought about by the substantial holding time at the extremely low temperature is not a chemical change, but rather a physical redistribution of particles within the material. The process reduces the susceptability of the materials treated to stress corrosion which is intergranular corrosion brought about by external and internal stresses and involves the internal breakdown of bonds between the crystalline structures of the metal which, obviously, will result in failure.

In a specific series of photomicrographs made of both a 316L stainless steel and a Haynes 21 cobalt base alloy, microstructure characteristic changes were observed which support the proposition that the method of the invention produces a better distribution of carbides present in the metal. In one particular group of the photomicrographs made over the same area of a sample before and after treatment, a substantial streak of accumulated carbides was observed before treatment and found to be broken and distributed after treatment. In another sample of the same stainless steel, some quite large carbides were found to be broken into a number of smaller carbides and there was a precipitation of the carbides to the grain boundaries, thereby showing both a breaking up and a movement of the carbides with other evidence indicating a redispersion of the base alloying media of the steel. Additionally, such photomicrographs gave evidence of a partial transformation from austenite to martensite in the grain structure of the metal. In a sample of the Haynes 2l cobalt base alloy, the photomicrographs evidence an improved dispersion of the smaller carbides.

The practical effect of the improved microstructure resulting from the method of the invention has been demonstrated by treatment of numerous tools which have evidenced substantially greater durability after treatment. Stainless steel razor blades have remained sharp for substantially longer periods of time after being treated by the method of the invention.

In addition to the benefits which have been found to derive from the application of cooling metals to extremely low temperatures by cryogenic fluids, it has also been found that materials such as nylons and other synthetics including polyurethane are substantially improved by application of cryogenic cooling. One particular example is ladies nylon hose, which have been treated by the same methods applied to metals with the result that their wearing characteristics and resistance to runs and snags have been drastically altered so that they last substantially longer. The same results have also been found in various valve parts made of polyurethane which were treated at the same time that the metal valve parts were treated.

The method of the invention has been described as preferably practiced with liquid nitrogen. It is to be understood that other cryogenic fluids may be employed. The liquid nitrogen is particularly preferred because of its relatively low cost and ease of handling, though it is to be recognized that an essential requirement for a cryogenic fluid in the method of the invention is that it be inert so that there will not be reaction with the material being treated by the fluid. For this reason, obviously, oxygen in the liquidified form is not considered desirable, particularly with those materials which readily oxidize. By the same reasoning, it is not recommended that cryogenic fluids such as hydrogen be used due to the extreme danger involved. Helium, of course, has a much lower boiling point, near the absolute zero level, though its cost for most purposes is prohibitive.

What is claimed and desired by Letters Patent is:

I. A method of treatment of materials consisting of metals by cryogenic cooling for improvement of the microstructure of such material comprising the steps of: reducing the temperature of said material to about 320F. by supporting said material above a body of liquid nitrogen in a container and lowering said material toward said nitrogen at a rate less than the rate at which thermal fracturing of said material occurs, immersing said material into said liquid nitrogen for a predetermined period of time within the range of about 18 hours to about 30 hours; separating said material and said liquid nitrogen; and permitting said material to return to room temperature, wherein the step of reducing the temperature of said material is effected at temperature and time rates of reduction varying from a minimum range of about 2.8-4F per minute to a maximum range of about l0.7l5F per minute in accordance with the mass and nature of said material and is effected in at least two stages above and below a predetermined thermal fracture threshold for said material.

2. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said step of reducing the temperature of said material is effected in three temperature range stages in accordance with two thermal fracture temperature levels for said material;

3. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said material is an AISI M2 steel, said material is reduced in temperature in a first stage at a rate within the range of about 7.7-l0 F. per minute to about l00 F. and in a second stage at a rate within the range of about 4.36 F. per minute to about 320 F., said material is maintained at about said 320 F. for about 24 hours, said liquid nitrogen is premitted to boil off, and said material is maintained in said fluid container until reaching room temperature.

4. A method of material treatment by cryogenic cool ing in accordance with claim 1 wherein said material is AISI M3 steel, said material is cooled in a first stage to a temperature of about lOO F. at a rate within the range of about 7. ll() F. per minute. said material is cooled in a second stage from about F. to about 250 F. at a rate within the range of about 3.55 F., said material is cooled in a third stage from about 250 F. to about 320 F. at a rate within the range of about 7.1-l0 F., said material is retained in said liquid nitrogen at about 320 F. for about 24 hours; said liquid nitrogen is permitted to boil off from said material, and said material is held in said container until said material reaches room temperature.

5. A method of material treatment by cryogenic cooling in accordance with claim I wherein said material is AISI M4 steel, said material is cooled in a first stage to about l00 F. at a rate within the range of about 7.1-l0 F. per minute, said material is cooled in a second stage from about 100 F. to about 250F. at a rate within the range of about 4.3-6 F. per minute, said material is cooled in a third stage from about 250 F. to about 320 F. at a rate within the range of about 2.84 F. per minute, said material is retained in said liquid nitrogen for about 24 hours, said liquid nitrogen is premitted to boil off from said material, and said material is held in said container until said material reaches room temperature.

6. A method of material treatment by cryogenic cooling in accordance with claim I wherein said material is an A18] 81 steel said material is cooled in a first stage to a temperature of about 100 F. at a rate between the range of about l0.715" F. per minute, said material is cooled in a second stage from about 100 F. to about 320 F. at a rate within the range of about 7.1-l0 F. per minute, said material is held in said liquid nitrogen for about 20 hours, said liquid nitrogen is permitted to boil off from said material, and said material is held in said container until said material reaches room temperature.

7. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said material is a steel selected from the class consisting of AlSl S5 and AISI S7 steel, said material is cooled in a first stage to about F. at a rate within the range of about l0.7,-|5F. per minute and is cooled in a second stage from about l50F. to about 320F. at a rate within the range of about 7. l-l0F. per minute, said material is held in said liquid nitrogen for about 20 hours, said liquid nitrogen is allowed to boil from said material, said second material is held in said container until said material reaches room temperature.

8. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said material is selected from a class consisting of differentially hardened tool steels and bearing grades of steels, said material is reduced in temperature in a first stage at a rate of about 10F. per minute to about lOF., said material is held at about l 00F. for about 30 minutes, said material is reduced in temperature in a second stage at a rate of about F. per minute to about 250F., said material is held at about -250F. for about 1 hour, said material is reduced in temperature in a third stage at UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 3, 891,477 Dated June 24, 1975 Inventor(s) James W. Lance and Henry M. Jones It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 49, cancel "particles" and insert practices line 53, cancel "Of" first occurrence and insert or Column 2, line 23, "break" should read "breaking".

line 60, "nitgogen" should read "nitrogen. Column 3, line 2, "cryognic" should read "cryogenic".

line 30, cancel "lever" and insert level line 55, "intergranualr" should read "intergranular". line 56, cancel "intergranular" and insert may Column 8, line 61, after "material" insert and line 62, cancel "second". Column 10, line 3, insert and after "off,".

line 3, cancel "still.

Signed and Scaled this Tenth Day of August 1976 {sum-l Anest:

RUTH C. MASON C. MARSHALL DANN Am'm'ng Officer Commissioner of Patent: and Trademark: 

1. A METHOD OF TREATMENT OF MATERIALS CONSISTING OF METALS BY CRYOGENIC COOLING FOR IMPROVEMENT OF THE MICROSTRUCTURE OF SUCH MATERIAL COMPRISING THE STEPS OF: REDUCING THE TEMPERATURE OF SAID MATERIAL TO ABOUT -320*F. BY SUPPORTING SAID MATERIAL ABOVE A BODY OF LIQUID NITROGEN IN A CONTAINER AND LOWERING SAID MATERIAL TOWARD SAID NITROGEN AT A RATE LESS THAN THE RATE AT WHICH THERMAL FRACTURATING OF SAID MATERIAL OCCURS, IMMERSING SAID MATERIAL INTO SAID LIQUID NITROGEN FOR A PREDETERMINED PERIOD OF TIME WITHIN THE RANGE OF ABOUT 18 HOURS TO ABOUT 30 HOURS; SEPARATING SAID MATERIAL AND SAID LIQUID NITROGEN; AND PERMITTING SAID MATERIAL TO RETURN TO ROOM TEMPERATURE, WHEREIN THE STEP OF REDUCING THE TEMPERATURE OF SAID MATERIAL IS EFFECTED AT TEMPERATURE AND TIME RATES OF REDUCTION VARYING FROM A MINIMUM RANGE OF ABOUT 2.8*-4*F PER MINUTE TO A MAXIMUM RANGE OF ABOUT 10.7*-15*F PER MINUTE IN ACCORDANCE WITH THE MASS AND NATURE OF SAID MATERIAL AND IS EFFECTED IN AT LEAST TWO STAGES ABOVE AND BELOW A PREDETERMINED THERMAL FRACTURE THRESHOLD FOR SAID MATERIAL.
 2. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said step of reducing the temperaTure of said material is effected in three temperature range stages in accordance with two thermal fracture temperature levels for said material.
 3. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said material is an AISI M2 steel, said material is reduced in temperature in a first stage at a rate within the range of about 7.7*-10* F. per minute to about -100* F. and in a second stage at a rate within the range of about 4.3*-6* F. per minute to about -320* F., said material is maintained at about said -320* F. for about 24 hours, said liquid nitrogen is premitted to boil off, and said material is maintained in said fluid container until reaching room temperature.
 4. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said material is AISI M3 steel, said material is cooled in a first stage to a temperature of about -100* F. at a rate within the range of about 7.1*-10* F. per minute, said material is cooled in a second stage from about -100* F. to about -250* F. at a rate within the range of about 3.5*-5* F., said material is cooled in a third stage from about -250* F. to about -320* F. at a rate within the range of about 7.1*-10* F., said material is retained in said liquid nitrogen at about -320* F. for about 24 hours; said liquid nitrogen is permitted to boil off from said material, and said material is held in said container until said material reaches room temperature.
 5. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said material is AISI M4 steel, said material is cooled in a first stage to about -100* F. at a rate within the range of about 7.1*-10* F. per minute, said material is cooled in a second stage from about -100* F. to about -250*F. at a rate within the range of about 4.3*-6* F. per minute, said material is cooled in a third stage from about -250* F. to about -320* F. at a rate within the range of about 2.8*-4* F. per minute, said material is retained in said liquid nitrogen for about 24 hours, said liquid nitrogen is premitted to boil off from said material, and said material is held in said container until said material reaches room temperature.
 6. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said material is an AISI S1 steel said material is cooled in a first stage to a temperature of about -100* F. at a rate between the range of about 10.7*-15* F. per minute, said material is cooled in a second stage from about -100* F. to about -320* F. at a rate within the range of about 7.1*-10* F. per minute, said material is held in said liquid nitrogen for about 20 hours, said liquid nitrogen is permitted to boil off from said material, and said material is held in said container until said material reaches room temperature.
 7. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said material is a steel selected from the class consisting of AISI S5 and AISI S7 steel, said material is cooled in a first stage to about -150*F. at a rate within the range of about 10.7*-15*F. per minute and is cooled in a second stage from about -150*F. to about -320*F. at a rate within the range of about 7.1*-10*F. per minute, said material is held in said liquid nitrogen for about 20 hours, said liquid nitrogen is allowed to boil from said material, said second material is helD in said container until said material reaches room temperature.
 8. A method of material treatment by cryogenic cooling in accordance with claim 1 wherein said material is selected from a class consisting of differentially hardened tool steels and bearing grades of steels, said material is reduced in temperature in a first stage at a rate of about 10*F. per minute to about -100*F., said material is held at about -100*F. for about 30 minutes, said material is reduced in temperature in a second stage at a rate of about 5*F. per minute to about -250*F., said material is held at about -250*F. for about 1 hour, said material is reduced in temperature in a third stage at about 5*F. per minute to about -320*F., said material is held at about -320*F. for about 20-24 hours, said liquid nitrogen is permitted to boil off, said still material is held in said container until reaching room temperature. 